That's such a great story!
It never occurred to me that you could do a mail merge with JS. What about just getting the pages offset printed, and then run them through a mail merge in something like Word or Libre Office, feeding the paper through the printer with the backside up?
Sure, there are probably many ways to have fixed their problem. But I knew Acrobat Pro well and remembered enough JavaScript to kludge a fix.
Likely not the most elegant solution, but way better than what they were doing before.